1. Field of the Invention
The present invention relates to an apparatus and method for limiting double exposure in a stepper. More particularly, the present invention relates to an apparatus and method which detect whether a shutter in the stepper is tightly closed when an exposing process stops, and limits double exposure by preventing further operation on the same or different wafers when the shutter is not tightly closed.
2. Description of the Related Art
During the normal fabrication of a semiconductor device, a wafer repeatedly passes through processes such as cleaning, diffusion, photolithography and ion-implantation. Each process requires a separate apparatus. Photolithography is a technique to form a mask image as a pattern of photoresist on a wafer. Photolithography consists of four steps: coating the wafer with a photosensitive material, aligning the wafer and exposing it to light patterned by the mask, developing the exposed photosensitive material on the wafer, and inspecting the wafer. The alignment and exposure are carried out in a stepper. Processing effectiveness in photolithography depends on how precisely the mask image is aligned on the wafer, i.e., how accurately the image is projected onto a given location on the wafer, and precisely how long the photoresist on the wafer is exposed.
In a stepper, the exposure principle is similar to that of a camera. That is, a light beam generated from a light source passes through a shutter depending on whether the shutter is opened or closed. The light passing through the shutter is then passed through a mask, focused, and projected onto a device area of a wafer, exposing the photosensitive material there in the mask pattern. The light is then blocked as the shutter is closed, and the wafer is moved to a new position, ready for the exposure of the next device area. The process is repeated for every device area on the wafer that is to be exposed. Thus, the wafer is alternately exposed to the light beam through the mask when the shutter is open and advanced to a new position when the shutter is closed, so that the photosensitive layer on the wafer is selectively exposed in the device areas.
In a conventional stepper, exposure is controlled by timing. The opening and shutting times of the shutter are adjusted depending on exposure time required for the particular photosensitive material and the device being fabricated. However, in some cases the opening and shutting operations of the shutter may not be performed exactly as designed. Improper performance can be due to aging of the motor or other system failure, such as a shutter that does not completely open or close, or a shutter that has one or more blades that are bent out of phase.
FIG. 1 shows the shutter of a conventional stepper. A light beam from a light source proceeds to an optical system through a window 6 of the shutter. The shutter is opened or closed by rotating a blade 4 using a stepping motor 2. However, if the phase of blade 4 is displaced, window 6 is not exactly opened or closed. Then, even though the shutter should be closed by design and according to time, the window is not entirely blocked, and some light is passed by the shutter.
If the repeated sequence of exposure and wafer movement is carried out in a stepper whose shutter's opening and shutting operations are not properly performed, the wafer is not correctly exposed. Device areas of the wafer that should be exposed may not get enough light. Furthermore, when the wafer is moved or when stepper operations are halted for a few seconds when the shutter should be closed, undesigned exposure may occur, i.e., double exposure or overexposure may occur since the shutter is erroneously opened. The conventional stepper has no capability to detect such double exposure. Thus, the undesirable exposure is performed without knowledge of the operators, thereby producing large numbers of poor quality wafers.
A need exists, therefore, for an apparatus that will detect the occurrence of double exposure in a stepper. Furthermore, the apparatus should determine when undesired exposure is occurring and modify the stepper operation in response.